Applied Composite Materials (v.21, #2)

Experimental Investigation of the Strain Rate Dependent Behaviour of 2D Biaxially and Triaxially Reinforced Braided Composites by R. Böhm; A. Hornig; J. Luft; M. Becker; I. Koch; B. Grüber; W. Hufenbach (285-299).
The performance of 2D biaxially and triaxially reinforced braided carbon fibre composites under dynamic loading is evaluated in the presented study. The accurate manufacturing of tensile specimen made of braided sleeves is explained particularly with regard to efficiency and reproducibility. In order to determine reliable strain rate dependent properties, the high-speed testing procedure is discussed. Using five materials, the parameter identification is described and relevant material data is provided. The measured stiffnesses and strengths are used to predict the non-linear stress-strain behaviour with an earlier proposed phenomenological damage model for textile composites. The gained orthotropic property-profile provides the input parameters for a numerical analysis of braided composite components using the calibrated model.
Keywords: Textiles; Non-linear behaviour; Damage mechanics; Strain rate dependency

Effect of Polymer Form and its Consolidation on Mechanical Properties and Quality of Glass/PBT Composites by R. T. Durai Prabhakaran; Saju Pillai; Samuel Charca; Simin Ataollahi Oshkovr; Hans Knudsen; Tom Løgstrup Andersen; Jakob Ilsted Bech; Ole Thybo Thomsen; Hans Lilholt (301-324).
The aim of this study was to understand the role of the processing in determining the mechanical properties of glass fibre reinforced polybutylene terephthalate composites (Glass/PBT). Unidirectional (UD) composite laminates were manufactured by the vacuum consolidation technique using three different material systems included in this study; Glass/CBT (CBT160 powder based resin), Glass/PBT (prepreg tapes), and Glass/PBT (commingled yarns). The different types of thermoplastic polymer resin systems used for the manufacturing of the composite UD laminate dictate the differences in final mechanical properties which were evaluated by through compression, flexural and short beam transverse bending tests. Microscopy was used to evaluate the quality of the processed laminates, and fractography was used to characterize the observed failure modes. The study provides an improved understanding of the relationships between processing methods, resin characteristics, and mechanical performance of thermoplastic resin composite materials.
Keywords: Thermoplastic composites; Polybutylene terephthalate (PBT); Vacuum consolidation; Compression properties; Prepregs; Fractography

Finite Element Analysis of Thermo-Mechanical Properties of 3D Braided Composites by Li-li Jiang; Guo-dong Xu; Su Cheng; Xia-mei Lu; Tao Zeng (325-340).
This paper presents a modified finite element model (FEM) to investigate the thermo-mechanical properties of three-dimensional (3D) braided composite. The effective coefficients of thermal expansion (CTE) and the meso-scale mechanical response of 3D braided composites are predicted. The effects of the braiding angle and fiber volume fraction on the effective CTE are evaluated. The results are compared to the experimental data available in the literature to demonstrate the accuracy and reliability of the present method. The tensile stress distributions of the representative volume element (RVE) are also outlined. It is found that the stress of the braiding yarn has a significant increase with temperature rise; on the other hand, the temperature change has an insignificant effect on the stress of the matrix. In addition, a rapid decrease in the tensile strength of 3D braided composites is observed with the increase in temperature. It is revealed that the thermal conditions have a significant effect on the strength of 3D braided composites. The present method provides an effective tool to predict the stresses of 3D braided composites under thermo-mechanical loading.
Keywords: 3-Dimensional reinforcement; Mechanical properties; Thermal properties; Finite element analysis (FEA)

A Stochastic XFEM Model to Study Delamination in PPS/Glass UD Composites: Effect of Uncertain Fracture Properties by D. Motamedi; A. S. Milani; M. Komeili; M. N. Bureau; F. Thibault; D. Trudel-Boucher (341-358).
A nonlinear extended finite element (XFEM) modeling framework under a stochastic cohesive zone is presented for realistic prediction of delamination in polyphenylene sulfide (PPS)/glass composites in mode I of fracture. The cohesive zone model adopts damage evolution of the material based on a bilinear traction-separation law, the critical energy release rate and the J-integral method to formulate the delamination interface under stochastic fracture properties. To demonstrate the application of the approach, numerical predictions are compared to experimental data using Double Cantilever Beam (DCB) tests. In particular, it is shown how the XFEM model can be used to capture test non-repeatability due to uncertain fracture properties, which is often the case during the characterization of composites using standard fracture tests.
Keywords: Polymer-matrix composites; Fracture toughness; Crack; Stochastic extended finite element

Investigations on Buckling Behaviour of Laminated Curved Composite Stiffened Panels by N. Jeevan Kumar; P. Ramesh Babu; Ratnakar Pandu (359-376).
In Industrial applications structural efficiency is primary concern, this brings about the need of strong and lightweight materials. Due to their high specific strength, fibre reinforced polymers find wide application in these areas. Panels made of composite materials are widely used in aerospace structures, automobile, civil, marine and biomedical industries because of their good mechanical properties, impact resistance, excellent damage tolerance and also low fabrication cost. In this Paper, buckling and post-buckling analysis was performed on composite stiffened panel to obtain the critical load and modes of failures, with different parameters like ply-orientation, different composite materials, and stiffeners and by changing the number of stiffeners was derived. To analyze the post buckling behaviour of composite stiffened panels the nonlinear finite element analysis is employed and substantial investigations are undertaken using finite element (FE) model. Effect of critical parameters on buckling behaviour is studied and parametric studies were conducted with analytical tool to understand the structural behaviour in the post buckling range.
Keywords: Composites; Pre-buckling; Post-buckling; Stiffened panel; Finite element

High Velocity Impact Response of Composite Lattice Core Sandwich Structures by Bing Wang; Guoqi Zhang; Shixun Wang; Li Ma; Linzhi Wu (377-389).
In this research, carbon fiber reinforced polymer (CFRP) composite sandwich structures with pyramidal lattice core subjected to high velocity impact ranging from 180 to 2,000 m/s have been investigated by experimental and numerical methods. Experiments using a two-stage light gas gun are conducted to investigate the impact process and to validate the finite element (FE) model. The energy absorption efficiency (EAE) in carbon fiber composite sandwich panels is compared with that of 304 stainless-steel and aluminum alloy lattice core sandwich structures. In a specific impact energy range, energy absorption efficiency in carbon fiber composite sandwich panels is higher than that of 304 stainless-steel sandwich panels and aluminum alloy sandwich panels owing to the big density of metal materials. Therefore, in addition to the multi-functional applications, carbon fiber composite sandwich panels have a potential advantage to substitute the metal sandwich panels as high velocity impact resistance structures under a specific impact energy range.
Keywords: Carbon fiber; Impact behaviour; Finite element analysis (FEA)